2.2 Dosing Regimens, Days Supply & Pediatric/Geriatric Calculations

Dosing Regimens, Days Supply & Pediatric/Geriatric Calculations

In Canadian pharmacy practice, ensuring the safety of a medication regimen involves verifying that the prescribed dose, frequency, and duration are clinically appropriate for the patient's age, weight, and organ function. Pharmacy technicians must master dosing conversions and days supply calculations to ensure therapeutic safety and accurate provincial or third-party drug insurance billing.

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Dosing Regimens & Frequency Interpretation

A dosing regimen specifies the quantity of drug (dose), the route of administration, and the dosing frequency. Technicians must understand standard prescription Latin abbreviations while upholding ISMP Canada standards that discourage ambiguous notations.

Frequency Abbreviations & Daily Multipliers

Liquid Volume Dosing

Liquid oral suspensions and solutions are expressed in concentration ratios (e.g., $125\text{ mg} / 5\text{ mL}$, $250\text{ mg} / 5\text{ mL}$, $400\text{ mg} / 5\text{ mL}$). To calculate the volume per dose:

$$\text{Volume per Dose (mL)} = \frac{\text{Dose Ordered (mg)}}{\text{Concentration on Hand (mg)}} \times \text{Volume of Hand Concentration (mL)}$$ $$\text{Total Volume to Dispense (mL)} = \text{Volume per Dose (mL)} \times \text{Doses per Day} \times \text{Number of Days}$$

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Days Supply Calculations

The days supply indicates how long a dispensed quantity of medication will last when taken exactly as directed. In Canadian community practice, entering an inaccurate days supply can lead to third-party audit clawbacks, premature refill rejections, or patient under-dosing.

1. Solid Oral Dosage Forms (Tablets and Capsules)

$$\text{Days Supply} = \frac{\text{Total Number of Units Dispensed}}{\text{Units per Dose} \times \text{Doses per Day}}$$

PRN Rule: For "as needed" prescriptions (e.g., "1 to 2 tablets PO Q4-6H PRN pain"), calculate days supply using the maximum possible daily consumption (Maximum Daily Dose, MDD):

• Maximum single dose = 2 tablets
• Maximum frequency = every 4 hours = 6 times per day
• Maximum daily dose = $2 \times 6 = 12\text{ tablets/day}$
• If 60 tablets are dispensed: $\text{Days Supply} = 60 / 12 = 5\text{ days}$.

2. Metered-Dose Inhalers (MDIs)

Inhalers are manufactured with a specific number of metered sprays (actuations) listed on the packaging (e.g., 120 or 200 actuations):

$$\text{Days Supply} = \frac{\text{Total Labeled Actuations}}{\text{Puffs per Dose} \times \text{Doses per Day}}$$

Worked Example: A patient is prescribed Flovent HFA 125 mcg (fluticasone propionate) MDI, containing 120 metered actuations. Sig: 2 puffs BID.

• Daily puffs: $2 \times 2 = 4\text{ puffs/day}$.
• Days supply: $120 / 4 = 30\text{ days}$.

3. Ophthalmic and Otic Drops

Pharmacy calculations conventionally use 20 drops per millilitre (20 gtt/mL) for eye and ear drops unless the question or the product gives a different drop size. Real droppers vary, so use the factor the question gives:

$$\text{Total Drops in Container} = \text{Bottle Volume (mL)} \times 20\text{ drops/mL}$$ $$\text{Days Supply} = \frac{\text{Total Drops in Container}}{\text{Drops per Administration} \times \text{Administrations per Day} \times \text{Number of Affected Eyes/Ears}}$$

Important Abbreviations:

• OD (oculus dexter): Right eye
• OS (oculus sinister): Left eye
• OU (oculus uterque): Both eyes

Worked Example: Latanoprost 0.005% ophthalmic solution, one 2.5 mL bottle. Sig: 1 drop OU once daily at bedtime.

• Total drops: $2.5\text{ mL} \times 20\text{ drops/mL} = 50\text{ drops}$.
• Daily drops: 1 drop $\times$ 2 eyes (OU) $\times$ 1 time/day = 2 drops/day.
• Days supply: $50 / 2 = 25\text{ days}$.

4. Insulin Calculations & Priming Units

Most insulin products in Canada are U-100 (100 units/mL). Concentrated insulins also exist, such as insulin glargine 300 units/mL (Toujeo), insulin lispro 200 units/mL (Humalog 200 KwikPen) and regular insulin U-500. Always read the concentration on the pen or vial. For U-100 products:

• 10 mL Vial = $10\text{ mL} \times 100\text{ units/mL} = 1,000\text{ units}$.
• 3 mL Cartridge or Prefilled Pen = $3\text{ mL} \times 100\text{ units/mL} = 300\text{ units}$.
• Box of 5 Pens (5 x 3 mL) = $1,500\text{ units}$.

Pen Priming (Safety Test) Rule: Manufacturer instructions for most insulin pens (e.g., KwikPen, SoloSTAR, FlexTouch) direct a 2-unit priming dose (safety test) before each injection to confirm needle flow and clear air. Some pens specify a different amount, so check the pen's instructions. When a question states the priming amount, add it to each injection when calculating days supply.

$$\text{Total Daily Insulin Usage} = (\text{Dose Units per Injection} + 2\text{ Priming Units}) \times \text{Injections per Day}$$ $$\text{Days Supply} = \frac{\text{Total Units Dispensed}}{\text{Total Daily Insulin Usage}}$$

Worked Example: A patient injects Humalog (insulin lispro U-100) 18 units TID before meals. The pharmacy dispenses one box of five 3 mL prefilled pens (1,500 units total).

• Usage per injection: $18\text{ units (dose)} + 2\text{ units (prime)} = 20\text{ units}$.
• Daily usage: $20\text{ units} \times 3\text{ injections/day} = 60\text{ units/day}$.
• Days supply: $1,500\text{ units} / 60\text{ units/day} = 25\text{ days}$.

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Pediatric Weight-Based Dosing

Children have immature organ systems, varying metabolic rates, and distinct body composition. Most pediatric drug dosages are calculated based on body weight in kilograms ($\text{mg/kg/day}$ or $\text{mg/kg/dose}$).

Step-by-Step Weight-Based Calculation Workflow

1. Convert weight from pounds to kilograms by dividing by 2.2.
2. Calculate the total daily dose (mg/day): $$\text{Total Daily Dose (mg)} = \text{Weight (kg)} \times \text{Dose Rate (mg/kg/day)}$$
3. Calculate the single unit dose (mg/dose): $$\text{Single Dose (mg)} = \frac{\text{Total Daily Dose (mg)}}{\text{Doses per Day}}$$
4. Convert the single dose to liquid volume (mL): $$\text{Dose Volume (mL)} = \frac{\text{Single Dose (mg)}}{\text{Concentration (mg/mL)}}$$
5. Safe Dose Verification: Always compare the calculated dose against the manufacturer's established maximum safe pediatric dose range. If the prescribed dose exceeds the maximum recommended limit, the technician must bring this to the pharmacist's immediate attention before dispensing.

Body Surface Area (BSA) Calculations

For antineoplastic agents, potent immunosuppressants, and critical care therapies, Body Surface Area ($\text{m}^2$) correlates more reliably with cardiac output and glomerular filtration rate than body weight alone. In Canada, the Mosteller Formula is the clinical standard:

$$\text{BSA (m}^2\text{)} = \sqrt{\frac{\text{Height (cm)} \times \text{Weight (kg)}}{3600}}$$

Worked Example: A pediatric patient is 120 cm tall and weighs 25 kg. Calculate the BSA:

$$\text{BSA} = \sqrt{\frac{120 \times 25}{3600}} = \sqrt{\frac{3000}{3600}} = \sqrt{0.8333} \approx 0.91\text{ m}^2$$

If the prescribed antineoplastic dose is $50\text{ mg/m}^2$, the dose is: $50\text{ mg/m}^2 \times 0.91\text{ m}^2 = 45.5\text{ mg}$.

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Geriatric Dosage Considerations

Elderly patients experience progressive physiological decline, including decreased renal blood flow, reduced glomerular filtration rate (GFR), reduced liver enzyme activity, and loss of lean muscle mass. Because serum creatinine (SCr) is generated by muscle breakdown, an elderly patient with low muscle mass may have a deceptively "normal" serum creatinine level (e.g., $70\ \mu\text{mol/L}$) despite having clinically significant renal impairment.

Cockcroft-Gault Creatinine Clearance (CrCl) Equation

In Canadian hospital and ambulatory clinical practice, creatinine clearance is calculated using laboratory SI units ($\mu\text{mol/L}$ for serum creatinine):

$$\text{CrCl (mL/min)} = \frac{(140 - \text{Age}) \times \text{Weight (kg)} \times 1.23}{\text{Serum Creatinine (}\mu\text{mol/L)}} \times (0.85\text{ if female})$$

(Note: When serum creatinine is reported in conventional US units of mg/dL, the formula is: $\text{CrCl} = \frac{(140 - \text{Age}) \times \text{Weight (kg)}}{72 \times \text{SCr (mg/dL)}} \times [0.85\text{ if female}]$).

If the calculated CrCl is less than $50\text{ mL/min}$ or $30\text{ mL/min}$, renally eliminated drugs (such as gabapentin, enoxaparin, ciprofloxacin, or novel oral anticoagulants) require downward dosage adjustments or extended dosing intervals to prevent toxic drug accumulation.